The mass ratio of air-to fuel (A/F) admitted to a diesel engine or a lean-burn gasoline engine is typically in the range of about 20–50 for good fuel economy. As a result the exhaust has a relatively low content of unburned hydrocarbons and carbon monoxide (typically about 200–700 parts per million, ppm, of hydrocarbons, and 500–1500 ppm of carbon monoxide) and a relatively low content of nitrogen oxides such as NO and NO2. These nitrogen oxides are referred to collectively as NOx and a typical NOx content in a diesel exhaust is about 50–500 ppm. Due to the oxygen content of the fuel-lean exhaust, unburned HC and CO can be oxidized in a catalytic oxidation converter. But reduction of NOx to nitrogen in the oxygen-containing exhaust has been a challenge.
Flow-through traps have been used to momentarily absorb NOx from the fuel-lean exhaust. A reductant, such as a small quantity of diesel fuel is then briefly introduced into the inlet of the lean NOx trap to provide reactive species for the catalytic reduction of the absorbed NOx. The NOx trap may be an extruded cordierite honeycomb structure with hundreds of cells per square inch of cross-section extending from the inlet face to the outlet face of the extrusion. The walls of the cells may be coated with a washcoat of high surface area alumina particles which are impregnated with an absorbent for NOx such as barium carbonate (BaCO3), an oxidation catalyst such as platinum, and a reduction catalyst such as rhodium and platinum. The finely divided platinum in the washcoat promotes oxidation of NO, to NO2. NO2 is absorbed from the lean exhaust by reaction with BaCO3 to form Ba(NO3)2 (barium nitrate) with the release of CO2 into the exhaust. When reductant species, such as CO, HC and/or H2, are introduced into the exhaust flowing through the cellular trap, NOx is released from the Ba(NO3)2 (with regeneration of BaCO3) and reduced to N2 over the rhodium and platinum particles in the washcoat.
Reductant species have been added to the exhaust by briefly injecting more fuel into the cylinders of the engine to a much lower A/F of 12–14. For example, the engine is operated in its normal fuel-lean mode for sixty seconds while NOx is stored in the LNT, and then operated in a fuel rich-mode for three seconds to provide reductant species for NOx removal and reduction. This practice requires more complicated control of engine fuel injection and causes noticeable variation in engine torque. Further, it permits NOx and HC emissions while the trap is being regenerated.
Reduction species have also been added directly to the exhaust stream, upstream of the inlet of the LNT, during a trap regeneration period. While this practice minimizes or avoids changes in engine operation, it still interrupts the trapping function of the LNT and permits NOx emissions during each trap regeneration period. Thus, there is a need for an improved practice for regeneration of the LNT.